The long-term goal of this project is to define a revised model of the functional anatomy of the basal ganglia which specifies the role of acetylcholine and its interaction with dopamine in the striatum. Current models emphasize effects of dopamine on the direct (striatonigral) and indirect (striatopallidal) projections and the role of striatal efferents in regulating the thalamus and the motor cortex and have provided significant insights into the pathophysiology of movement disorders, particularly Parkinson's disease. However, important gaps remain in the understanding of striatal function. In spite of the clinical utility of anticholinergic drugs in Parkinson's disease, the mechanisms by which acetylcholine modifies striatal output remain largely unknown. This proposal will test the principal hypotheses that dopamine and acetylcholine exert opposing influences on striatonigral and striatopallidal neurons, mediated by specific receptor subtypes. The proposed experiments will investigate the following Specific Aims: 1) to establish the cellular basis for functional interactions of dopaminergic and muscarinic cholinergic receptors in the striatum; 2) to demonstrate direct muscarinic actions on striatal projection neurons; and 3) to demonstrate antagonistic interactions between dopaminergic and muscarinic effects on striatal projection neurons. To accomplish these aims, receptor colocalization will be examined using in situ hybridization and selective cholinergic effects on neuropeptide expression in striatal projection neurons will be examined in striatal slices using radioimmunoassay and in situ hybridization. Selective lesions of cholinergic interneurons and the 6-hydroxydopamine model of Parkinson's disease will be used to evaluate effects of loss of acetylcholine or dopamine on striatal neurons. In addition to providing insight into the cellular mechanisms by which two neuromodulators can regulate a common neuronal substrate, the proposed model has direct implications for pathophysiology and potential therapeutic interventions in neurodegenerative diseases such as Parkinson's disease that affect the basal ganglia.